1. Field of the Invention
This invention relates generally to the field of circuit boards. In particular, this invention relates to substrates for circuit boards comprising a polybutadiene/polyisoprene thermosetting composition and an ethylene propylene rubber.
2. Brief Description of the Related Art
Circuit boards generally comprise an electrical substrate material laminated to a metal layer, usually copper. Electrical substrate materials used in circuit boards are most commonly composites, comprising a polymeric matrix and an inorganic filler. A variety of polymeric matrices have been used in these composites substrate materials, for example, thermoset resins together with inorganic particulate and/or fiber filler. Polybutadiene and polyisoprene resins and combinations thereof are particularly useful thermosetting compositions as described in commonly assigned U.S. Pat. No. 5,223,568 to Landi et al. and U.S. Pat. No. 5,571,609 to St. Lawrence et al., both of which are herein incorporated by reference in their entirety.
U.S. Pat. No. 5,571,609 discloses a thermosetting resin comprising a polybutadiene or polyisoprene resin and an unsaturated butadiene- or isoprene-containing polymer in an amount of 25 to 50 percent (vol %); a woven glass fabric in an amount of 10 to 40 vol %; and a particulate filler in an amount of 5 to 60 vol %. This filled composite material leads to a prepreg which has very little tackiness and can therefore be easily handled by operators. The material in accordance with this patent has a low dissipation factor, a consistent, low dielectric constant, and a low coefficient of thermal expansion, and is therefor ideal for wireless communication and high speed, high frequency signal transmission applications.
Other polymeric matrices and substrates commonly used for circuit boards are disclosed in U.S. Pat. No. 5,264,065 to Kohm, which describes a base material for printed wiring boards where inert filler is used to control Z axis coefficient of thermal expansion in fiberglass reinforced thermoset resins. The Kohm patent discloses a range of 45-65 weight percent (wt %) fiberglass reinforcement and a range of 30 to 100 parts filler per 100 parts of the polymer. U.S. Pat. No. 4,997,702 to Grant et al. discloses a circuit laminate having an epoxy resin system which also includes inorganic fillers or fibers in the range of 20-70 wt % of the total composite. The fibers include both glass and polymeric fibers and the fillers include clay or mineral (e.g., silica) particulate fillers. U.S. Pat. No. 4,241,132 to Pratt et al. discloses an insulating board comprising a polymeric matrix such as polybutadiene and a filler consisting of polymeric filler (e.g., fibrous polypropylene). In all cases, the dielectric constant or dissipation factor of the resin matrix is matched to the fibrous reinforcement in order to obtain an isotropic composite.
European Patent No. 0 202 488 A2 discloses a polybutadiene based laminate wherein a high molecular weight, bromine-containing prepolymer is used to reduce tack and flammability of a 1,2 polybutadiene resin. Similarly in Japanese Patent No. 04,258,658, a high molecular weight compound is added to a tacky polybutadiene resin to control tack. The compound utilized is a halogen-containing bismaleimide which provides flammability resistance as well as good copper bonding and heat resistance. There is no mention of the use of fillers and the resulting laminate has a relatively high dissipation factor.
The article entitled "1,2 Polybutadiene-High Performance Resins for the Electrical Industry", by R. E. Drake, ANTEC '84 pp. 730-733 (1984), generally discloses conventional polybutadiene resins for use in laminates and specifically discloses the use of reactive monomers which react with the polybutadiene. U.K. Patent Application 2 172 892A, generally discloses laminates composed of styrene-containing and thermoplastic copolymers with unsaturated double bonds and polybutadiene.
While the above composites are well-suited for their intended purposes, the industry constantly strives for circuit boards having improved dielectric strength, i.e., the resistance of a dielectric, an insulator, to electrical breakdown under the influence of strong electric fields. Such circuit boards have better reliability under extreme conditions.
Prior art attempts to improve dielectric strength focussed on lowering the degree of cure of the resin since it was thought that resin shrinkage was associated with reduced dielectric strength. However, such lower degree of cure, while it increased dielectric strength, also led to the undesirable effect of increased solvent absorption, increased warpage of the panels, and increased dissipation factor. Consequently, there is a perceived need in the art for improving the dielectric strength while maintaining optimal electrical, chemical, and thermal properties, as well as low cost.